At the present time wavelength division multiplexing technologies are used to increase transmission capacity of fiber-optic communication systems. There are two main fields of WDM application—dense wavelength division multiplexing (DWDM) and coarse wavelength division multiplexing (CWDM). DWDM technologies are mainly used in long-haul telecommunication systems and CWDM technologies are used in metro and access networks.
The DWDM is a high-capacity optical-transport technology, but its price is rather high. An international standard wavelength grid, suggested by the International Telecommunication Union (ITU), provides the following frequency separations: 200, 100, 50 or 25 GHz (wavelength spacing of 1.6, 0.8, 0.4, and 0.2 nm respectively); systems with higher wavelength division (12.5 GHz) are already used. For CWDW systems, the number of wavelengths that can be transported on a single fiber is less than that for DWDM systems, the channel spacing, recommended by ITU, is 20 nm. CWDW technologies are simple to use and low-cost.
In nodal points of fiber-optic communication systems for adding/dropping communication channels, optical add/drop multiplexers (OADM) are usually used. They make it possible to drop one or several channels from the line and at the same time to add the signal at the same wavelengths with new information. This provides a significant increase of effective use of communication systems. OADM with fixed channel frequencies are limited in their ability to drop wavelength and add one to the networks by nature of the fixed-wavelength transmitters deployed. Systematically growing demands for increased capacity of communication systems and the use of new approaches require greater flexibility than fixed (static) add/drop devices provide, complicating network operation and planning.
Use of dynamically reconfigured and tuned optical add/drop multiplexers (ROADM or t-OADM respectively) removes this constraint by allowing any channel to be dropped/added at any time, providing optimal routing in hubs along the networks. Besides, t-OADM may also be used in wavelength division systems, where wavelengths may change.
Construction ROADM, well-known to specialists in the sphere of optical systems, is assembled from discrete components including demultiplexers, switches and multiplexers. Typical multiplexers and demultiplexers include multilevel structures on thin filters, diffraction gratings in free space optics or arrayed waveguide gratings for guided wave optics (AWG). Optical switches, used for dropping, adding and passing channels, are, as a rule, microelectricmechanical systems (MEMS). The difficulty with this conventional approach is that it is needlessly expensive, especially if the number of channels in the system is high. It is characterized by significant input losses and degradation of optical signal quality. Besides that, optical switches are susceptible to environmental effects, such as temperature changes and vibration.
The main functional element of t-OADM is a tunable filter—a wavelength selective optical component in which the central wavelength of the selected bandpass can be tuned dynamically. There are many tunable optical filters, but most of them, for various reasons cannot be used successfully in t-OADM. For instance, tunable filters based on acoustic-optical effect are polarization dependent, which causes many practical problems. The Bragg filter is tuned mechanically or by a resistive heater, and tuning speed is therefore comparatively slow—typically of the order of millisecond. Tunable filters based on Fabry-Perot interferometers are also largely unacceptable, since it is not feasible to achieve the necessary degree of fineness in these filters; if they can tune the WDM range, they do not have narrow enough channels, and if they have narrow enough channels, they can only tune a portion of required band.
The tunable filter based on asymmetric or unbalanced/nonsymmetrical Mach-Zehnder interferometer (further, single-stage MZI), is characterized by a low signal insertion loss, low polarization dependence, relatively low cost. Equipped with electro-optic phase shifter, it can provide extremely fast tuning. It is known to those skilled in the art that multi-stage structure on the base of asymmetric single-stage MZI with 8 or 9 stages is characterized by a high selectivity and is sufficient to cover the entire WDM band. That is why among all tuneable filters, specified above, this tunable filter is a more attractive choice for application in t-OADM and ROADM.
There is a controllable add/drop optical multiplexer (U.S. Pat. No. 6,795,654, B2), that includes an input port, a drop port, an output port, and an add port and includes means for providing an input signal consisting of channels to the input port, a plurality of filter stages connected to the input port, each filter stage operating to selectively transmit either even or odd channels and reflect either odd or even channels respectively, means for providing reflected channels as a pass signal at the output port, and means for providing a transmitted channel at the drop port. Each filter stage could comprise a fiber unbalanced MZI having a selective delay for transmitting the selected channels and a mirror for reflecting channels not transmitted by the fiber unbalanced MZI. The means for providing reflected channels as a pass signal at the output port and the means for providing an add signal at the add port, such that the add signal follows the reverse path of the drop signal, could comprise circulators.
Use of this tunable multiplexer provides realization of the method of selectively passing and dropping channels from an input signal (U.S. Pat. No. 6,795,654, B2), the method comprising the steps of: selectively transmitting either even or odd channels and reflecting either odd or even channels respectively; this operation is repeated as many times, as it may be necessary to reflect all the channels, except the desired channel; providing a transmitted channel as the drop signal at the drop port and combining of reflected channels at the drop port, providing an add signal at the add port and combining the add signal and the pass signals.
The scheme of one of embodiments of such multiplexer—device 10—is shown on FIG. 1. Multiplexer 10 has an input port 11, an output pass+add port 12, an output drop port 13, an add port 14 and includes three one-stage MZIs: 15-1, 15-2 and 15-3, formed with the help of three coupler pairs {16-1, 16-2}, {16-3, 16-4} and {16-5, 16-6} and, as interferometer arms, connecting optical fibers {17-1,17-2}, {17-3,17-4} and {17-5,17-6}. The difference in optical path of arms in three interferometers increases in two times during transfer to the next interferometer.
Each of three specified single-stage MZI 15-1, 15-2 and 15-3 transmits the selected channels and with the help of fiber-optical mirrors 15-1-1, 15-2-1 and 15-3-1 reflects and returns back even or odd channels respectively. Two signal routing components are used: circulator 18-1, connected with add 11 and pass 12 ports, meant for providing an input signal consisting of channel to the input port 11  for providing reflected channels as a pass signal at the output pass+add port 12, and circulator 18-2—for transmitting of channel at the output drop port 13 and for providing an add signal at the add port 14 such that the add signal follows the reverse path of the drop signal.
The three-stage structure provides drop of one channel during add of 8 channels at the add port and add of the new channel in spite of dropped one. Tunable phase shifters 15-1-2, 15-2-2 and 15-3-2, installed in one of the arms of every of three interferometers 15-1, 15-2 and 15-3 respectively, are used for controllable tuning of spectral characteristics of the specified one-stage MZIs 15-1, 15-2 and 15-3 and in that way for add/drop of any of eight channels.
According to the patent (U.S. Pat. No. 6,795,654, B2), other proposed variants propose to make single-stage MZI with the help of discrete elements: splitters, mirror-prisms, polarizer and modernized Layot filters. As an alternative to mirrors 15-1-1, 15-2-1 and 15-3-1 and circulators 18-1 and 18-2 it is possible to use an additional structure of optical filters on single-stage MZI for transmitting of channels to output add/pass port 12.
The device specified above provides add and drop any of eight channels of an optical network. However, the device suffers from several essential downsides.
It is known to specialists in the sphere of optical networks that structure, described above, contains a lot of optical elements—one-stage MZI in fiber or discrete versions, mirrors and circulators, and is rather large-dimensioned and cannot be reliable and stable in real conditions, because one-stage MZI is very susceptible to environmental effects, such as temperature changes, vibrations and other factors. That is why the realization of such devices requires another approach, using integral-optical technologies. It is also known that one-stage MZIs transmissions are characterized by the non-flatness in the passband and also the narrowness of the stopband, which may cause a crosstalk and low isolation between adjacent channels. Besides that, one-stage MZIs cause undesirable large dispersion into a filtered channel, that during rapid transmission causes pulse widening data signal and so a decrease in the transmission capability of the optical network.
It is known that two-stage unbalanced MZIs or multi-stage unbalanced MZIs (further two-stage and multi-stage MZIs) have significantly better spectral characteristics and lower dispersion, but these devices are not bidirectional and so cannot be used in add/drop multiplexer 10, described above.
To provide possibility of integral-optical construction of controllable optical add/drop multiplexer, it is necessary to reduce the amount of optical elements used and eliminate circulators and mirrors, because they are not compatible with integral-optical technology. Reduction of the amount of optical elements used is also rational from a financial point of view (the price of the device will be cheaper).
Therefore the creation of methods of a controllable add/drop and of a controllable optical add/drop multiplexer that is less engineered and meets present requirements concerning channel insulation and dispersion, and may be executed in an integral-optical version, is urgently needed. It is desirable for such a device to have additional functional opportunities, to be maximally dynamic and flexible enough, i.e. to provide the best ratio of technical characteristics to price for various applications.